A previous article from this laboratory (Maron & Adelstein, 1976) reported no significant differences in the cardiac myosins of patients with obstructive hypertrophic cardiomyopathy and normals based on purified myocin ATP'ase activity and light chain composition by one-dimensional SDS-polyacrylamide gel electrophoresis. Since that report, several aminal models of cardiac hypertrophy have shown the presence of myosin isoenzymes, due to variants in heavy chain structure. Hoh and associates, using a technique of pyrophosphate gel electrophoresis, demonstrated myosin isoenzymes in the rat which change in distribution with age and with the impaired contractile state of hypothyroidism. Flink and Morkin, in the rabbit, used 2-dimensional mapping of cyanogen bromide digests of myosin to demonstrate a new myosin isoemzyme in hypertrophied rabbits. Schwartz and her associates demonstrated a similar isoenzyme shift in the rat with hypertrophy induced by aortic banding.